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Rhododendrons, and the closely related azaleas and are terrific for introducing bursts of color into the garden throughout the spring and summer. More so with rhodies as these have larger bell-shaped flowers protruding from the end of the stems, and each of those have ten or more stamens.
Rhododendrons are often labelled as easy to care for plants. That, they are, but only when they’re planted in ideal conditions. If anything is off with the growing conditions, this is one shrub that’s not very forgiving.
It’s likely to lack the strength to fend off the diseases they’re prone to succumbing to, some of which include pest attacks, diseases on the leaves and fungal diseases that affect the roots.
Rhododendron Growing Checklist
Rhododendrons need acidic soil with a pH of 5.5 or less.
The soil you use needs to be porous, and well-draining, too. You can test your soil drainage by digging a 12” x 12” hole, filling it with water, letting it drain, then refilling it and timing it the second time you fill the hole with water. It should drain by about 1” per hour.
If yours is draining faster, it’s likely too dry, therefore would benefit from more organic material being added to improve moisture retention. Slower drainage is more of a problem because that would mean the roots would be sitting in standing water, putting them at risk of root rot.
If you’re growing rhododendrons in a container, you can add up to 20% perlite or pumice to improve drainage. For rhododendrons that have been transplanted into the ground, a sprinkling of elemental sulfur (not aluminum sulfur) around the top of the soil will eventually acidify it once the rainwater washes it into the soil mix.
How to Correctly Acidify the Soil for Rhododendron
Some methods of acidifying soil in a bid to try to save a dying rhododendron can actually kill it. Aluminum sulfate is one of the most harmful as it’s toxic to the roots. So is lime.
Should your rhododendron be planted too close to paved pathways, or too close to a building, there could be lime leaching into the soil causing the pH to increase.
Best practice for planting rhododendrons is to have it at least 5-feet away from a building’s foundations. However, it’s safer to err on the side of caution by planting it at least 5-feet away from any other structure that could cause lime to leach and alter the soil acidity.
If you have a soil with a pH over 7.5, it’s going to be really hard to lower it substantially, in which case, container growing rhododendrons would be the easier alternative, bypassing a lot of guesswork.
Nutrient Competing Plants Are Any with Shallow Roots
Rhododendrons have shallow roots and they can easily be out-competed by other plants and trees. As they need dappled sunlight, the solution is often to plant it under a tree so it has some canopy to shade it from the hot afternoon sun.
That can work, unless it’s a maple tree, which has shallow roots too so those will compete with rhododendrons for nutrients. Walnut trees are worse as those produce toxins that can kill rhododendrons so if you have yours planted near a walnut tree, it will need to be moved.
Rhododendrons grow best when they have 4 to 6 feet of space between them and other plants with similar shallow roots. If your design calls for companion plants, choose plants with a deeper root system that won’t be competing for the same nutrients in the topsoil around rhododendrons.
- Soil acidity: Under 5.5 pH (if the pH is above 7.5, grow in a container, or a raised garden bed).
- Drainage: Porous soil that drains at an average 1” per hour. Soil amendments may be needed.
- Surrounding plants should not have shallow roots as those compete for nutrients in the soil.
- Rhododendrons transplanted in the ground should be at least 5-feet away from buildings and other structures that can leach lime into the soil.
- If you’re growing more than one rhododendron, put 4 to 6 feet of space between them so as not to restrict air circulation.
Learning how to save a dying rhododendron is only beneficial if you can give it the conditions it needs. Most problems that effect rhododendrons are because of one or more of their preferred growing conditions are not being met.
As examples, early signs of root rot are likely to show as wilting and yellowing on the leaves as a result of improper soil drainage. Likewise, fungal infections are encouraged when there’s insufficient air flow caused by grouping two or more rhododendrons or other shallow-rooted plants too close together.
Check your growing conditions against the checklist summary above to see if your plant would need to be moved to a more preferable location, or if there’s amendments that need to be made to the soil. When the growing conditions are rectified, most problems with rhododendron shrubs can be fixed with care over time.
6 Common Diseases Rhododendrons Are Affected by When the Growing Conditions Aren’t Ideal
1 – Bud Blast
Bud blast makes your flowers look terrible. They turn brown and dry up, then by next spring when you expect to see blossoms, you wind up with ugly black bristly-spores instead.
Bud blast is a fungal infection that kills flower buds and if left untreated, it can spread to branches and travel throughout the plant.
Leaf hoppers are the cause. The buds on rhododendrons have slits that leaf hoppers use to lay their eggs. When they do, they introduce a fungal infection. Remove the dead flowers, and any drying branches around damaged flowers.
In the spring to late in the summer is when leaf hoppers are active. The best, and probably only effective preventative measure is to apply a coating of diatomaceous earth around the flowers.
2 – Leaf Gall / Flower Gall
Rarely will you find a growth-like tumor on a plant to be good news. Surprise! It is in this case. Leaf gall just looks super unhealthy on any plant or infected flower, but it will not kill a rhododendron. It’s caused by a fungus called Exobasidium vaccinii.
In its early stages, the leaf turns a paler green color, then later, as the weather gets more humid, the infected leaf swells and develops a white coating over all or part of the leaf.
Those are fungal spores and they spread. The spores can overwinter and re-emerge next spring, so any infected parts should be trashed. Don’t compost any leaves or flowers with galls.
3 – Petal Blight
When your rhododendrons develop freckles on its petals, it’s probably petal blight caused by Ovulinia fungus. This is a nasty one. Early stages are small brown freckles on white petals and on colored petals, the spots will be white.
Small to start with, they swell, soften, flowers rot, stick to the leaves and it does not stop. The fungus spreads swiftly.
If caught early enough, you can nip it in the bud by discarding infected parts of the plant. The fungus will survive in the soil though so it’s best to replace ground mulches with fresh organic materials and if you’re growing in containers, replace the top layer of potting soil.
Once a rhododendron becomes a host plant, care needs taken to prevent it recurring. Do that by keeping a layer of mulch about 4” thick and thin it out nearer the trunk.
If it does become an ongoing problem, treat it with a fungicide before buds start opening using a product that contains chlorothalonil, or captan. Both of those kill Ovulinia fungus spores, but it’ll need to be done before the buds open.
4 – Dieback and Root Rot caused by Phytophthora
Phytophthora cinnamomi causes root rot and phytophthora cactorum causes dieback. They are both soil-borne pathogens that can lay in dormancy for years, then when the conditions are just right, they spring into action and kill the roots of everything in their path.
Phytophthora root rot can kill a rhododendron in under two weeks. Early signs of dieback include chlorosis (yellowing leaves), leaf curl, lesions on the leaves, and leaf curling inward before eventually dropping.
The symptoms of root rot caused by Phytophthora are much more difficult to spot because the pathogen enters through finer roots, spreading to the trunk, then throughout the rest of the shrub.
Younger plants die much faster, but mature rhododendrons can show gradual signs of wilting, drooping and chlorosis for up to a year before dying.
The only way to tell if a rhododendron has phytophthora in the soil is to have the soil tested. If you suspect it to be a problem, you can slow the spread by removing the plant from the soil and letting the roots dry out.
Phytophthora needs specific conditions to become a threat. Those are moist acidic soil that drains poorly. Four hours of standing water is all it takes for phytophthora to germinate.
Fix the soil conditions, and next time, try growing a phytophthora resistant rhododendron.
5 – Cankers Caused by Rhizoctonia or Botryosphaeria
Rhizoctonia web blight is mostly visible on the leaves when humidity and temperatures are high. If you look at the brown areas on an infected plants’ leaves, you’ll see a brown webbing covering them. That’s the rhizoctonia fungus. The leaves mat tightly together holding them under the canopy, then when temperatures drop below 70oF (12oC), they fall off.
To confirm a rhizoctonia fungal infection, take a look at the roots of the plant near the soil line for brown lesions or cankers. Rhizoctonia infections start near the soil-line with a brown or reddish-brown lesion that eventually enlarges forming a canker.
A canker grows and can girdle around the stem/trunk restricting nutrient uptake from the soil. The drought effect is most noticeable with wilting in midday when temperatures are higher.
Rhizoctonia is soil-borne, but unlike Phytophthora, it isn’t quick to kill rhododendrons. It is damaging, but not fatal if treated with a fungicide. Pruning some of the branches, twigs, and leaves to increase air flow will help too.
Botryosphaeria cankers have similar symptoms, but unlike rhizoctonia, web blight on the leaves won’t be present. Instead, leaves will wilt and die, as will parts of the branches and twigs.
Botryosphaeria enters wood through wounds in the bark, especially with cuts made with tools that weren’t disinfected to prevent the spread of infections from other plants. The only way to save a rhododendron from certain destruction from Botryosphaeria cankers is to cut away dead and dying tissue late into its dormancy period but shortly before the buds open. In most species, that’s early spring.
The severity of infection will determine how much pruning needs to be done. If it’s severe with multiple dying branches causing poor foliage and flowering, it can be cut down to 6” above ground level. It will take a few years to recover though.
Up to 15” can be sufficient. As you’re cutting the branches back, pay attention to the main branches extending from the crown. There are usually more than three primary branches, then multiple off-shoots.
Prune the off-shoots first as those will grow back faster than the main branches. If the canker is on a main branch, make the cut a few inches below it to make sure only healthy wood is growing back in.
6 – Cercospora Leaf Spot
Cercospora leaf spot is a fungal infection that causes brown spots to cover leaves. The discoloration is darker toward the outside turning a lighter brown in the center, and if you look closely in the center of the spot, you’ll see a tiny dark pimple. That’s the fruiting part of the fungus that produces spores causing the infection to spread to surrounding leaves.
The best treatment is to remove the leaves, and get rid of any leaves that have dropped to the ground. A spray fungicide can be applied to the leaves to prevent the fungus from re-emerging.
How to Save a Dying Rhododendron from Most Fungal Infections in 4 Steps
- Remove any diseased foliage and dead or dying branches and twigs.
- Apply a fungicide treatment that contains chlorothalonil, or captan.
- If transplanted in the ground, consider if potting is possible. If it’s too big to move, an alternative is to replace most of the soil surrounding the roots and trunk and apply a new layer of organic mulch to about a 4” thickness. Container grown rhododendrons should have their soil changed.
- Prune and deadhead to encourage new growth faster and to improve air circulation, which lowers humidity levels too. Much of the fungal infections affecting rhododendrons happen when humidity levels soar above 90%.
The only fungal infection a rhododendron (or any neighboring plants) cannot be saved from is Phytophthora, unless you catch it super early, which is unlikely as it attacks the roots, and stems from the inside. All other fungal diseases can be treated with the removal of infected parts, and if required, repeated fungicide applications.